Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a prototypical black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, referred to herein as the REaD 101 process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. The photoreceptor is then recharged, exposed, and developed for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various color toner particles are placed in superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
The REaD IOI process can be implemented in various ways. For example, in a single pass printer wherein the composite final image is produced in a single pass of the photoreceptor through the machine. A second implementation is in a four pass printer, wherein only one color toner image is produced during each pass of the photoreceptor through the machine and wherein the composite color image is transferred and fused during the fourth pass. REaD IOI can also be implemented in a five cycle printer, wherein only one color toner image is produced during each pass of the photoreceptor through the machine, but wherein the composite color image is transferred and fused during a fifth pass through the machine.
Whatever the implementation, the photoreceptor is initially charged for the first exposure and then recharged for subsequent exposures. One important consideration following recharge is the voltage between previously developed toner layers and the photoreceptor. If the voltage is too large undesirable interactions occur between toner layers, resulting in poor color separations.
However, in REaD IOI systems that use split recharging it has been found that print quality defects that are associated with low- or wrong-sign toner in the developed toner can occur. Two of these defects are under color splatter ("UCS"), in which development of a second color causes particles of the first color to jump into background areas, and cross-contamination, in which dislodged particles of the first color are pulled into the development housing of another color, and subsequently redeveloped. Both of these defects tend to become more objectionable when the REaD IOI system is optimized for more robust rendering of small lines and/or dots. Using an AC scorotron rather than a DC scorotron charging device at the second stage of split recharge generally helps improve latitude against these defects, but they might not be entirely eliminated.
An alternative to split recharge is direct AC recharging in which the photoreceptor is first erased (using flood exposure) after each color development step and then the photoreceptor is recharged using a high-slope AC device. The AC device, although predominately delivering ions of the charging polarity, will produce an increasing level of opposite-polarity ions as the target voltage is approached. Those ions serve to reduce toner voltage, but are not so numerous as to produce excessive UCS and cross-contamination defects. However this approach depends on the use of photoreceptor erasure to assure upwards charging in all photoreceptor areas. Because the erase device requires physical space and because it may require a minimum time before recharge (to enable the photoreceptor to recover from the effects of the high light levels employed) this may not be practical, particularly in single-pass REaD IOI architectures.
Therefore a recharge approach which controls toner layer voltage without creating an objectionable degree of cross-contamination or under color splatter, and which does not require the use of photoreceptor erasure would be beneficial.